Method and apparatus for measuring the direction sense of direction and energy of ionizing processes



Sept. 8, 1959 FEARQN EI'AL 2,903,594

METHOD AND APPARATUS FOR MEASURING THE DIRECTION SENSE 0F DIRECTION ANDENERGY OF IONIZING PROCESSES Ongmal Filed July 50, 1949 7 Sheets-Sheet 1INVENTORS ROBERT E. FEARON JEAN M. THAYER @KM A a R 1!!! viii 9/1! 91/4MIMI It!!! Illa n M m A B .m H R 4 7 s M 7 LIT R G m w u P M A w u C\\\\m MU JV b ....c... m R .H 3; ms m m Sm E .m w n a Pwfi D 0 ATTORNEYSept. 8,

T0 SURFACE R. E. FEARON ETAL METHOD AND APPARATUS FOR MEASURING THEDIRECTION SENSE OF DIRECTION AND ENERGY OF IONIZING PROCESSES OriginalFiled July so, 1949 IMPEDANCE A MATCH V37 SHAPER I I in SCALING 35 in/AMP.\/34

f J DISCRIM- AJ INATOR 33 32 AMF. |-F 3| W 29' I l 26 l -74-] i '76! l16 24 SOURCE OF NEUTRONS DEPTH 7 Sheets-Sheet 2 A FUNCTION OF INTENSITY6%} INVENTORS ROBERT E. FEARON JEAN M THAYER MA? W ATTORNEY Sept. 8,1959 FEARON ETAL 2,903,594

METHOD AND APPARATUS FOR MEASURING THE DIRECTION SENSE OF DIRECTION ANDENERGY OF IONIZING PROCESSES ori inal Filed July so, 1949 7 Sheets-Sheet3 (I60 I61 (I62 uZ m 2' gt: y u: m7 .1 8 32 1.1 0 .J(0' m 58 g 2 3 E W30 I63 0 U U DETECTOR b a: z! 0! a t: -u 2 0 3E fl- I a: [1. o 2 m U zmu a mg 5 2 U 73 a: E DETECTOR (2 m INVENTORS ROBERT E. FEARON JEAN M.THAYER ATTORNEY Sep t'."8;v1 ,9 R. E. FEARON ETAL 2,903,594

METHOD'ANDAPPARATUS FOR MEASURING THE DIRECTION SENSE OF DIRECTIONANDENERGY OF IONIZING PROCESSES Original Filed July 30, 1949 V 7Sheets-Sheet 4 3+ PHASE 5+ INVERTER DETECTOR s TO PULSE HEIGHT SELECTORTO ELECTRODE 2 2 2 J DELAY 26O AM P Ll NE AMP. DERIVATOR THRESHOLDCIRCUIT g m AMI? SHAPE R F. 3 1' 3 zaa 289 290 at? 9 INVENTORS ROBERT E.FEARON JEAN M. THAYER gin M ATTORNEY Sept. 8,

dri inal Filed July so, 1949 1959 R. E. FEARON ETAL 2,903,594

METHOD AND APPARATUS FOR MEASURING THE DIRECTION SENSE OF DIRECTION ANDENERGY OF IONIZING PROCESSES 7 Sheets-Sheet s 230 g 23' 3 0. O. T 2|7 gn M 6' 2 E225 U 228 I m -4-U n 0- 3 -0: M 30!!! 11 5 2 a; O.(U I I I 220222 22s 224 225 O (2') C+ J 0: "I'- i- LIJ 0.. g d753 3z3 m 227 u 1 u0:! a. g g 7 5 & Io -o' Ll?- 292 D. l 234 ARTIFICIAL LINE HIGH PASS HIGHPASS FILTER T0 PULSE DCONDITION- 5 LING EQUIP- MENT INVENTORS ROBERT E.FEARON JEAN M. THAYER ATTOR N EY Sept. 8, 1959 R. E. FEARON ETAL2,903,594

METHOD AND APPARATUS FOR MEASURING THE DIRECTION SENSE OF DIRECTION ANDENERGY OF IONIZING PROCESSES Original Filed July 50, 1949 7 Sheets-Sheet6 v L I At fzfl V E J l INVENTORS At ROBERT E. FEARON By JEAN M. THAYERkm ATTORNEY Sept. 8, 1959 R. E. FEARON ETAL 2,903,594

METHOD AND APPARATUS FOR MEASURING THE DIRECTION SENSE OF DIRECTION ANDENERGY OF xomzmc PROCESSES Original Filed July 30, 1949 '7 SheetsSheet 7301 303 DIFFERENTIATOR 9 r d r r 6* PULSE -THRESHOLD Ia |PPERDIFFERENTIATO INPUT AME 312 313 3 .L r 3|4 c- DELAY 3|5 31s f C+- wv C-5+ 6+ 5+ L? cc- 5 3w OUTPUT C l 76 76 1 l i 320 x r i I l \a I iINVENTORs ROBERT E.FEARON JEAN M. THAYER A TTORNEY Patented Sept. 8,1959 fiice METHOD AND APPARATUS FOR MEASURING THE DIRECTION SENSE FDIRECTION AND ENERGY OF IONIZING PROCESSES Robert E. Fearon and Jean M.Thayer, Tulsa, Okla,

assignors to Well Surveys, Inc., a corporation of Delaware Originalapplication July 30, 1949, Serial No. 107,806,

now Patent No. 2,712,081. Divided and this application April 19, 1954,Serial No. 424,104

21 Claims. (Cl. 250-8345) This invention relates generally to a methodand apparatus for identifying substances existing in difiicultlyaccessible locations, for example, adjacent to a deep narrow drill hole,and more particularly to a method and apparatus for identifying anddistinguishing these substances from each other by nuclear reactions inthe substances. This is a division of our copending application SerialNo. 107,806, filed July 30, 1949, now Patent 2,712,- 081, for a Methodand Apparatus for Neutron Well Logging.

This invention is directed to the solution of a problem which has beenlong recognized by geologists and geophysicists, and by others,confronted with the problem of locating valuable substances, such aspetroleum, in the sub-surface formation of the earth. The problem ofdiscovering with certainty the existence of a particularly valuablesubstance in the subsurface formations of the earth has only beenpartially solved by the prior art workers. All prior efforts to solvethe problem have met with failure for the reason that no parameter couldbe found which was solely characteristic of the valuable substances thatit was desired to locate. As an example, in the art of well logging apartial solution to the problem goes as far as determining withcertainty that either salt water or petroleum exists in a particularformation but a complete solution is not possible, since prior to thisinvention, no parameter was known whereby the two substances could bedistinguished, in situ, from each other.

Numerous other methods advanced by the workers in the prior art forlocating valuable substances in the subsurface formations of the earthinclude: electrical methods which involve the measurement ofself-potential, conductivity, and resistivity; thermal methods; seismicmethods which treat of the acoustical properties of the sub-surfaceformations; natural radioactivity of the formations; and those methodsin which the formations are irradiated with radioactive radiations andan effect such as the gamma radiation produced by the neutroninteractions in the formations measured. All of these methods as well asothers which have not been enumerated above, have not afforded acomplete solution to the above problem in that none of them measures aparameter that is solely characteristic of the valuable substances thatone is desirous of locating.

For the purpose of particularly describing and setting forth andrespects in which this invention differs from and represents advancementupon the prior art, there is set forth a description of the efforts ofprevious workers insofar as they have been directed to the problem whichhas been stated in the previous paragraph.

The location of petroleum has been attempted by various well loggingmethods which are sensitive to some physical characteristic imparted tothe rocks by the presence of petroleum in them. For example, resistivitymethods in combination with other methods somewhat ambiguously enabledetection of petroleum. The inconvenience and uncertainty of the use ofresistivity methods arise from the fact that resistivity is a generalproperty of rocks, and is possessed by some rocks not containingpetroleum to an even greater extent than the degree to which theproperty is manifested by certain other rocks full of petroleum. Forexample, Indiana limestone will be found to have a much higherresistivity than oil saturated sandstone of the Frio formation in theGulf Coast oil fields. Furthermore, sandstone which contains naturalgas, has a high resistivity, as does also coal. Moreover, limestone mayshow a decrease of resistivity where an oil bearing horizon appears. Itcould similarly be shown how each and every one of the other nonnuclearlogging methods have specific shortcomings which analogously preventthem from being or amounting to a specific recognition of petroleum.

The instant invention provides a complete solution to the above problem.This solution consists of a system of observations by which the operatoris enabled to recognize and quantitatively measure nuclear species ofthe sub-surface formations adjacent a bore hole. Although the desiredsubstances quite often are not elements or single nuclear species thechemical laws of combining proportions enable accurate appraisal of suchthings as the occurrence of petroleum. Recognition of nuclear species isaccomplished by subjecting the substance adjacent to the bore hole tobombardment with penetrating radiations of a nature to cause specificand determinative quantized changes in the potential energy of the saidnuclear species. These quantized energy changes which are specific tothe particular kinds of atoms to be determined are used as a means ofrecognizing the desired atoms, which recognition is accomplished bymeans of selective neutron detection, selective for specific energyranges of neutrons, and/or specific limits of direction of incidence andsense of direction of incidence.

Among the means which are required for the solution of the aboveproblem, there is provided exceedingly powerful and energeticallyefficient monoenergetic neutron sources, relying upon the nuclearreactions caused by electrical or electromagnetically acceleratedparticles. These are provided in a form which is adapted to be loweredinto a bore hole, and employed therein to bombard the rocks adjacent tothe bore hole. Also required for the practice of this invention arepowerful capsuled neutron-emitting sources, depending for theiroperation upon energeticparticles emitted by radioactive substances.There is set forth the manner of choosing and designing suchneutron-emitting sources, showing how a person skilled in the art canavail himself of intensities hundreds of times greater than those whichare now available.

Required in the practice of this method are various means of observingneutrons which permit the determination of the energy, the direction ofincidence of neutrons, and the sense of direction.

Among these means, there are provided devices which determine bothenergy and direction of incident neutrons within certain limits. Thereis also provided a device for detecting phenomena described in nuclearphysics as n-p reactions. This device enables exact determination ofenergy of neutrons, and a somewhat ambiguous determination of direction.Incidental to the practice of this invention also is a device forresolving nuclear data which gives only a general indication of energy,and interpreting this general indication of the energy of neutrons in amore exact way. There is also provided, as a means of practicing thisinvention, a choice of the manner of employment of a number of neutronfilters adapted to select specific-energy groups of neutrons. It isshown how these filters may be employed for the purpose of identifyingspecific elements in the strata.

Therefore the primary object of this invention is the provision of amethod and apparatus for identifying valuable substances by separatelymeasuring the influence of specific properties. of the nuclei of thevaluble substances upon a flux of fast neutrons.

Another important object of this invention is the provision of a methodand apparatus whereby petroleum can be positively identified in thesubsurface strata adjacent a bore hole.

This invention also contemplates a method and means for locatingvaluable substances situated in difiiculty accessible locations byidentifying and measuring the mfluence of at least one of its elementarycomponents on a flux of fast neutrons.

Still another object of this invention is to achieve the above objectsby irradiating formations with fast neutrons and measuring the intensityof neutrons falling within specific energy bands and which haverebounded from the formations.

Another object of this invention is to provide a method and means forproducing a log of a drill hole by recording versus depth the averagerate of occurrence of processes occasioned by fast neutrons of selectedenergies which enter the detecting device.

A further object of this invention resides in the provision of a methodand means for detecting neutrons, selecting pulses produced therebywhose energies lie in a predetermined range, and recording theirtime-rate of occurrence versus depth.

Another object is to provide means for delivering to a recorderelectrical signals which denote the intensity of neutrons of a definiteenergy class.

This invention also contemplates means for determining specific energylosses in samples of substances exposed to neutrons of a determinedenergy for the purpose of adjusting energy selective neutron detectorsystems used in well logging.

Another object is to provide a method and means to accomplish deepinvestigation in a direction perpendicular to a bore hole andconcurrently provide very detailed resolution of thin strata.

Still another object is to provide detectors which are directionallysensitive and which are adjusted with respect to the source forfavorable angle of fast neutron scattering from formation substance thatare capable of determining variations of the properties of strata withdistance horizontally.

This invention also contemplates a novel detector whereby dipdeterminations can be made in a drill hole.

Another object of this invention is to provide a detecting systemwhereby horizontal anisotropy can be detected.

Still another object is to provide means for detecting horizontalanisotropy in a measurement based upon a particular energy of neutrons.

A further object is to provide a detecting system whereby verticaldirection sensitivity and sense of direction of neutrons of a particularenergy can be detected.

Another object is to measure the energy of pulses having a particularenergy.

Other objects and advantages of the present invention will becomeapparent from the following detailed description when considered withthe drawings, in which:

Figure l is a schematic illustration of a Well logging operaiton showingthe surface recording system;

Figure 2 is a diagrammatic illustration of a sub-surface instrument withthe detector illustrated in vertical section;

Figure 3 illustrates the type of well log that would be Produced by thepresent invention;

Figure 4 shows in perspective a vertical section of a novel detector;

Figure 5 is a schematic diagram of a fast neutron detectlng systemhaving transverse directional sensitivity;

Figure 6 illustrates in cross-sectional view a modification of thedetector shown in Figure 5;

' discussed above may be made.

. j 4 g s Figure 7 is a schematic diagram of a fast neutron detectingsystem having directional sensitivity and whereby pulses representingthe energy of ionizing processes may be utilized;

Figure 8 is a schematic diagram of another embodiment of fast neutrondetecting systems in which there are neutron directional sensitivity,sense of direction sensitivity, and energy sensitivity;

Figure 9 illustrates schematically a further embodiment of a detectingsystem in which direction, sense of direction, and energy of fastneutrons are detected;

Figure 10 is a wiring diagram illustrating the threshold circuit shownschematically in the bottom portion of Figure 9;

Figure 11 shows a system of graphs which illustrate the behavior of thecircuit of Figure 10;

Figure 12 is a schematic diagram of an electrical system for pulseduration selection; and

Figure 13 is a diagrammatic illustration of a detector, similar to thatin Figure 4, in which is shown the geometric interpretation of theapplication of the circuit shown in Figure 12.

As pointed out above, consideration of the problem of well logging hasled to the conclusion that there is a necessity for the discovery ofmethods which will identify more specifically the substances found inthe rocks atljacent to wells which are logged. Specific identifyingproperties, which could be relied upon as a means of recognition ofsubstances, must be able to cause an effect which is observable underthe logging conditions which prevail. Preferably the process making theobservations possible should be one which acts through space and throughmatter which fills the space between the position in which the rock tobe identified is found, and the location of the detecting apparatus inthe bore hole. The necessity for acting through space arises because ofthe prevalence of easing and/or cement and/or fluid of various sortswhich commonly exist in the well bores, and which interfere with themeasuring process. Another reason why considerable action through spaceis essential is the need for the depth of investigation to be adequate.Considerable depth of investigation is a highly desirable factor in welllogging because of the heterogeneity of rocks making shallowobservations unrepresentative, and therefore inaccurate as arepresentation of the whole mass of rock penetrated.

There are available at the present time only a very few types ofinfluences by which desirable observations as Obviously, the magneticand electric fluxes are not available for consideration in connectionwith cased wells, and the electric flux is unusable when investigatingnon-conducting material. The observation of the heat flux is familiar inthe art of well logging and has patently the disadvantage that such observations are slow if one desires a considerable depth ofinvestigation. The transmission of observable infrared and ultravioletradiations is excluded because of the opacity of substances generallypresent in the earth and in bore holes. The gravitational flux hassatisfactory properties, and, in principle, could be measured. But noknown means of measuring it for well logging purposes has been found.

In attacking the above problem, seeking for a method of specificrecognition of material in the circumstances of a bore hole penetratingthe rock strata of the earth, it has been discovered that there areapparent specific properties of atomic nuclei corresponding with energytransitions in those nuclei. These transitions may evidence themselvesin a variety of ways, such as:

a. The emission of radiant energy through space.

b. The absorption of a particular amount of energy from a bombardingparticle or quantum.

c. A specific energy threshold or a plurality of energy thresholds ofsusceptibility of the nuclei to certain classes '5 of nuclear change,which may be caused by bombarding corpuscles or quanta.

It has been discovered that in all branches of molecular, atomic, andsub-atomic physics, one may generally predict that if a specific energytransition is possible in a quantized system, there will be a resonanceeffect, specifically affecting bombarding particles or quanta possessingenergy (either kinetic or potential) in the close vicinity of the amountrequired to produce a quantized transition. The discovery of the detailsof quantization of nuclei of atoms still waits for extensiveexperimental and theoretical work. Limited experimental evidence hasalready brought support to the conviction which exists in the minds ofall nuclear physicists to the effect that nuclei will surely be found tobe quantized systems. In some instances energy thresholds of variouskinds have already been determined for nuclei. For example, thephoto-neutron threshold is now known experimentally through the study ofits inverse process, capture, by Kubischek and Dancoff.

A specific energy threshold at 20 mega volts has been found for thesystem comprising 4 nucleons (2 protons and 2 neutrons). Sundry isomerictransitions corresponding with highly forbidden transformations of thearrangements of nucleons have been found experimentally and can beconsidered as additional evidence of the truth and experimentalsignificance of the general conclusion that nuclear matter exists inquantized energy states.

In an effort to make use of the foregoing general conclusion, it hasbeen discovered that only two classes of radiation appear to exist whichreact with nuclear matter appreciably and can be arranged convenientlyfor the observation of quantized energy levels of nuclei. These classesof radiation are the photon or electromagnetic class, and thecorpuscular class comprising neutrons. Other particles (charged) ingeneral do not penetrate the coulomb field of force surrounding anucleus at energy falling in the range of possible excitation processesof common nuclei. Such excitation processes are typically expected forlight nuclei in the vicinity of 1 million electron volts.

Charged particles lack action through a distance as defined herein.Therefore, corpuscular radiations of the charged variety would, inprinciple, not be particularly useful for investigation of the quantizedlevels of nuclei. Of the classes of radiation which have been suggested,the only one which has been discovered which has a favorable ratio forthe amount of interaction which it undergoes with nuclear matter, ascompared with the energy transitions effected in the progress of theradiation by circumstances arising outside the nuclei of atoms, is theneutron. The photon reacts extensively with orbital electrons, and hasonly a very small cross section (target probability) for interactionswith nuclei as such. There is furthermore an additional reaction ofphotons which becomes prominent above 2 electron megavolts, and which,in the range above 2 electron megavolts results in materialization ofelectron-position pairs. This materialization, through influenced by thepresence in the near vicinity of the nuclear field of force, does notrepresent a specific or identifying characteristic of particular nuclei,but is a general characteristic of all nuclei, more prominent for thenuclei of light elements such as aluminum. For the above listed reasons,there appear to be only a few especially simple reactions caused byphotons which might be of any use. One might find it desirable toobserve the neutrons released from nuclei by photons, since there is,for such nuclear photo-neutrons, a specific threshold of energy for eachnuclear species (element or isotope thereof). One might also investigatethe unmodified Compton scattering of energetic photon radiations in thehope of finding some slightly modified lines which suffered loss ofenergy by interaction with nuclei. This possibility is somewhat favoredby the fact that the otherwise much stronger 6 modified Comptonscattering radiation is rapidly eliminated from the flux by absorption.

On the other hand, the interaction of neutrons with the outside, partsof the atom is so small that the direct production of ion-pairs byneutrons is found to occur on an average of only about one time permeter of ordinary atmospheric air for a neutron possessing a kineticenergy of five million electron volts. The liberation of energy byneutrons in air therefore amounts to less than one thousandth of 1% permeter of air traversed, for energy liberated by processes involving theoutside portions of the atoms found in the air. A distance of travel inair which would result in an average loss of energy by reaction withoutside parts of the atoms of less than 1%, would, nevertheless, resultin total absorption of the neutrons, and all their energy, by reactionwith the nuclei of the atoms contained in air. Even so, many of thereactions which neutrons undergo, which occur between neutrons andnuclei of the matter, are not highly specific, nor do they aid in anyrefined efforts to identify such matter. Among the unidentifying nuclearreactions one may name, for example, conservative ballistic nuclearscattering of neutrons, that is, conservative of total kinetic energy.This process is specifically different to an extreme degree only in thecase of very light elements such as hydrogen and helium. The averagenature of other matter contained in the rocks is sufficiently alike inthis respect that the main possibility of use of the property ofconservative ballistic nuclear scattering of neutrons is to observedifferences in the propagation of neutrons through the rock which enableconclusions regarding the presence of hydrogen to be made. This effectis already made use of, and there exist a considerable number of US.patents and other published descriptions bearing on this subject. Amongthese patents are No. 2,308,361, No. 2,220,509, and No. 2,349,712. Thebroad class under which these previously named inventions fallcorresponds with a patent issued to John C. Bender, No. 2,133,776.

The theory of detection of hydrogen by conservative ballistic nuclearscattering is treated in an article written by Robert E. Fearon andpublished in the June 1949 issue of Nucleonics, entitled Neutron WellLogging.

The above theory finds general application in pursuing this method, andFigures 1 and 2 more particularly set forth the details of arrangementsthrough which these general concepts find specific application to theproblem set forth above.

Referring to these figures there is illustrated an application of thisinvention to a well surveying system. In Figure 1 there is shownschematically a drill hole 10 which may or may not be cased. Disposed inthe drill hole and adapted to be raised or lowered therein is a housing11 supported by a cable 12. Cable 12 comprises at least one. electricalconductor connecting the electrical apparatus within the housing 11 toapparatus located adjacent the mouth of the drill hole 10. The apparatuson the surface of the earth consists of a measuring wheel 13 over whichthe cable 12 passes and a winch or drum 14 on which the cable is wound,or from which it is unwound, when the housing 11 is raised or lowered inthe drill hole 10. Conductors are connected to the cable 12 by means ofslip rings 15 and brushes 16 carried on one end of drum 14. Theseconductors lead to an amplifier 17. Amplifier 17 is a conventional audioamplifier having a fiat frequency response. The output of amplifier 17is conducted to a pulse shaper 18, the purpose of which is to insure thedelivery of square topped waves of constant height to an integrator 19.Integrator 19 is adapted to receive the aforementioned pulses andgenerate therefrom an electromotive force which is proportional to theaverage time-rate of occurrence of the pulses. This signal is deliveredto the recorder 20 where it is reconded versus depth. The depth axis ofthe recorder is actuated by the shaft 21 which leads from a gear box 722, connecting through shaft 23 the measuring wheel 13. The gear box 22has adjustments to enable suitable choice of depth scales.

Referring specifically to Figure 2 a description of the contents ofhousing 11 will follow. It is to be understood that housing 11 will beconstructed to withstand the pressures, and mechanical and thermalabuses encountered in surveying a deep bore hole and yet provideadequate space within it to house the necessary apparatus and permit thetransmission of radiation through it.

In the bottom portion of housing 11 there is located a radiation source24 which may be surrounded by a radiation filtering material 25. Thisradiation source may take various forms which will be described indetail later in the specification. Above the filtering material 25 andlying between the source of radiation and a radiation detector 26, thereis a region of space which may be occupied by suitable materials or leftvacant determined by considerations explained as the descriptionprogresses.

The detector 26 is of the type which will detect neutrons as a result ofthe production of prominent bursts of ionization therein, caused byrapid movements of heavy charged particles such as protons, alphaparticles, etc., set in motion by the neutrons. The bursts of ionizationare very quickly collected in the detector 26. These bursts areregistered as electrical pulses and resolved timewise from other orsmaller pulses which may occur almost concurrently. The detector 26 isso designed and so operated that the magnitude of the electrical pulsereleased from the collection of a specified amount of electrical chargewill always be quite accurately proportional to the amount of theelectrical charge collected and substantially independent of the path inthe detector along which the burst of ionization occurred.

The current corresponding to a pulse, flowing in the electrode circuitwhich includes conductor 27, resistance 28, battery 29 and conductor 30,produces a voltage pulse across the resistance that is of the formillustrated at a. The pulse produced across the resistance 28 isimpressed through the condenser 31 upon the input of an amplifier 32. Asshown at b the pulse has suffered negligible loss and no distortion inpassing through the condenser 31. The amplified pulse, illustrated at c,has been inverted in polarity but otherwise faithfully reproduced. It isthen conducted to the pulse height distribution analyzer 33. Here onlythose pulses whose magnitude fall within a prescribed range, such asillustrated at d and designated by e, are accepted and transmitted.Other pulses such as are illustrated at f and g are not accepted andtransmitted. Those pulses which are accepted and transmitted aredelivered to an amplifier 34. Amplifier 34 is one having a flatfrequency response extending upward to the highest frequency required tofaithfully amplify the pulse delivered to it in a manner shown at h. Theoutput signal from the amplifier 34 is fed into a scaling circuit 35which, in a known manner, delivers pulses as illustrated at i, thenumber of which, occurring in a given time is less by a constant factorthan the number received in the same interval of time. The output of thescaling circuit is fed into a shaper 36 which transforms the pulse intothe shape illustrated at i. The shaper 36 may take the form of apowdered iron core transformer. The signal from the transformer is thenfed into impedance matching means 37, such as a cathode follower, whichfaithfully reproduces the voltage wave as illustrated at k. Theimpedance matching means 37 introduces the signal into the transmissionline contained within the cable 12 for the purpose of transmitting it tothe surface.

It is to be understood that all elements within the housing 11 whichrequire power may be powered in a conventional manner as taught in theart by means such as batteries or rectified alternating current.Batteries Which very satisfactorily fulfill the temperature require- '8ments in hot wells are the zinc, potassium hydroxide, mercuric oxidecells.

Again referring to .Figure 1, the signals transmitted to the surface bymeans of cable 12 are taken therefrom by means of slip rings 15 andbrushes 16 and are conducted to the amplifier as pulses, one of which isillustrated at I. These amplified pulses are received by a pulse shaper18 which modifies their form in the manner illustrated at 0. The pulseillustrated at 0 will always have a fixed square form with a fixedheight m and a fixed width 11. These substantially square pulses arethen fed into the integrating circuit which delivers the signal to therecorder 20, as has been previously described. The integrating circuitthus produces a time-dependent voltage wave such as shown at p. Whenthis signal is impressed on the recorder, which has been coordinatedwith depth, a curve will be drawn as shown in Figure 3. This curve hasits ordinate depth in the bore hole and as its abscissa a function of anintensity of received radiation, or of a plurality, or combination ofintensities. These intensities may be combined by adding, subtracting,or dividing in any desired manner, or may be otherwise mathematicallycombined. The manner of combination is suitable to specificallyindicate, or be especially sensitive to, the presence of a particularsubstance in the region adjacent the bore hole.

Although no power supply has been shown in connection with the surfaceapparatus, it is to be understood that it will be powered in aconventional manner such as was pointed out in connection with thesub-surface apparatus.

This invention includes a particular type of radiation detector whichhas been invented and heretofore constructed and tested, together withcertain improvements which have also been discovered. This improvedradiation detector consists of an ionization chamber containing a gas ofvery low molecular weight, sufficiently low that, when a molecule oratom thereof is struck by a neutron, considerable of the kinetic energyof the neutron is likely to be transferred to the molecule or atom.Adequate pressure must be maintained in this detector, so that thenumber of atoms of the target substance contained in it will besufiicient per unit of volume for a small detector adapted for welllogging (of the order of 3% outside diameter) to have a usableefiiciency. It is contemplated in this respect that pressures will beemployed which will lie in the range of from one to several atmospheres,extending upward to the vicinity of 300 atmospheres. It has been foundthat, for the observation of electrical impulses which will be causedwhen the atoms of this con tained gas are struck by neutrons and proceedto liberate their energy by moving through the remaining portion of thegas it is necessary to choose a gas which does not, under the conditionsof the use of the detector, permit the formation of negative ions ofmolecular size from the electrons which are freed by the passage of therecoiling atom. The electrons are thus permitted to remain free, and maybe very quickly collected, distinguishably from the heavy ions ofpositive charge which remain, and which are set in motion very much moreslowly, and are relatively unobserved. Such a detector is illustratedgenerally in Figure 2 and more specifically in Figure 4.

As shown in both figures a housing 74 encloses a system of electrodesdisposed in an atmosphere of compressed helium. Electrode 75 in the formof a cylindrical plate is disposed in the bottom of, but insulated from,the housing 74. Ring-shaped electrodes 76 are arranged in spacedrelationship, one above another, and insulated from each other and fromthe housing. The axes of all these rings are coincident and coincidewith the axis of the housing. Each ring by itself may be said to liegenerally in a plane perpendicular to the axis of the housing 74. Thevertical distances between the successive planes pass ing through thecenters of successive rings, are equal. A screen 77 is situated at thetop of the system of rings and also lies in a plane perpendicular to theaxis of the detector. Screen 77 is electrically connected to andsupported by the inner surface of the housing 74 and divides the spacewithin the housing into two parts. The lower portioncontains the greaterpart of the internal volume. The portion of space above the-screen 77contains an electrode 27 which is maintained at a strong positivepotential with respect to the housing 74 by means of the battery 29which is connected to the resistance 28. The circuit is completed to thecase by virtue of the connection of the cathode of battery 29 to groundthrough the conductor 30. The electrode 75 and rings 76 are electricallyconnected to a battery 78, and coupled by equal resistors 79 in such amanner that the numerical value of the negative potential from the firstof these electrode elements 75 and 76, steadily and evenly decreasestoward ground potential referenced from the bottom electrode to the topelectrode inside the housing 74. The effect of these electrodes 75 and76, and of screen 77, is to produce a uniform straight electric field,directed downward along the axis in the greater part of the interior ofthe chamber, everywhere except in the close vicinity of the rings 76. Ifan ionizing path occurs in the helium below the screen in the region ofelectrode elements 75 and 76, the electrons thus released areimmediately translated upward toward the screen 77. Some are absorbed bythe screen and are lost, but a larger number, and a relatively large andconstant fraction of the total number pass through, and are attractedtoward the collector electrode 27, where they are collected, causing anelectrical impulse to occur in the external circuit, coupled throughcondenser 31. The pressure of the helium is chosen with respect to theuse which it is desired to make of the detector. The specific choice ofpressure in a given case will correspond with a value for which therange of the most energetic recoiling helium atoms will be appreciablyless than the diameter of the rings 76, but the range of electrons oflike energy will be much more than the diameter of these rings. Theneutron detector of Figures 2 and 4 may be employed for the detection offast neutrons, since slow, or thermal ones do not produce observablerecoil processes. As such, it is a detector which observes fastneutrons, and can be used to observe them in the presence of slow oneswithout being influenced by the slow ones. Also it is able to ignoregamma rays by the choice of pressure which renders inefficient theionization of its atmosphere by gamma rays. Thus, while gamma rays willcause pulses to be generated in this detector they Will be of lesserenergy per pulse, and may, therefore, be eliminated.

The detector of Figures 2 and 4 may also be used to detect slowneutrons, or to observe n-p reactions on substances which may be mixedwith the helium. All these uses of the detector of Figures 2 and 4enable determination to be made of the energies of the fast neutronsimpinging on the detector, as will be fully shown further on as thedescription progresses.

It has been found that the gases of chemical group zero do not formnegative ions when approached by electrons. Use has been made of thisfact in certain counters arranged for the measuring of alpha particles.Such counters have been constructed which use noble gases at atmosphericpressures or lower pressures. There is, contrary to previous theoreticalconsiderations, an appreciable rate of recombination in the noble gasesfor ions produced therein. It has been satisfactorily demonstrated thatsuch recombinations are not caused by third body processes or by walleffects, but are specific and inherent physical characteristics of thenoble gases themselves. These findings regarding the rate ofrecombination do not, nevertheless, exclude the use of these gases, atelevated pressures. On the other hand, Biondi has recently found that amuch higher rate of recombination prevails. for hydrogen. The pressurelimitation for the production of electron-caused quick pulses fromhydrogen will therefore be much lower than the corresponding limitationfor helium. Helium is preferred over hydrogen for use in this radiationdetector at elevated pressures. The recoil of helium from a neutronderives from a square hit roughly 75 of the kinetic energy of theneutron. In the case of a 10 million electron volt neutron, the recoilcould have nearly 6 million electron volts energy, and would correspondwith an alpha particle having a range approximately 6 centimeters instandard air. The liberation of energy by such an alpha particle instandard air would be about the same as it would be in helium at sevenatmospheres. The dimensions of a chamber in which such a recoil couldefficiently liberate its energy are therefore not excessive for anypressures above fourteen atmospheres of helium.

It is necessary to produce a pulse having a size which corresponds withthe number of electrons liberated by the recoil, and which always hasthe same size for alpha ray paths liberating the same number ofelectrons, but liberating them in different parts of the ionizationchamber. This result may be accomplished by an arrangement like thatdescribed in US. Patent No. 2,469,460. Pulses proportional to the numberof electrons initially released may also be produced by the use of arelatively fine wire for the center electrode and a sufliciently strongelectric field in the vicinity of the center electrode to bring aboutgas amplification, that is, an incipient avalanche, in the closevicinity of the fine wire. Also the neutron detector, as shown inFigures 2 and 4, accomplishes this purpose and it may be used as a pointproportional counter by connecting a suitably high voltage battery at29. Any of these methods will result in pulses which are proportional tothe number of electrons initially liberated and substantiallyindependent of the potential difference through which the electronsinitially liberated have fallen. if these precautions are not taken,pulses will occur in which the influence of each primary electron ismultiplied by a weighting factor which is the potential differencethrough which it falls in arriving at the collector electrode from thepoint at which it originated. Since the weighting factors which havebeen mentioned are not dependent upon the characteristics of theelectric field in the measuring apparatus, and since the use of theseweighting factors cause pulses absorbing the same amount of radiantenergy in the chamber to bring about the transfer of different amountsof energy into the electrical output, and since they also cause pulsesfor which different amounts of radiant energy were liberated in thechamber to effect transfer of equal amounts of electrical energy intothe output, on a chance basis; the use of an ordinary ionization chamberwill result in great confusion, and will prevent, or tend to prevent,the clear recognition and classification of nuclear phenomena as theyoccur in the ionization chamber. It is understood that in pursuing thismethod, it is intended to make use of such special detecting methods asare described herein. There are certain ways of practicing thisinvention which do not require the use of a special detector. Note shallbe taken of these exceptions, and it will be pointed out that thedetector may be more freely designed in such cases.

A quicker collection of electron charge is achieved in ionizationchambers which contain at least sufiicient diatomic or polyatomicmolecular gases to furnish means of dissipating the energy of electronswherever their energy becomes appreciably higher than the value thatwould correspond with equipartition of energy, under Maxwellianstatistics of the gas. Where such polyatomic molecules or otherdissipants of energy are not present, as for example, in an ionizationchamber filled exclusively with very pure helium, there is no mechanismwhich enables an electron to lose its energy at all efliciently unlessit has acquired energy more than that which corresponds with the firstquantized state above ground level for the extranuclear electron shellsof the helium atom. Since this first transition corresponds with a fewvolts, the electron gets out of true equilibrium with its environment,wherever it is able to derive kinetic energy from an electric field, andwhile its motion remains random in direction except for drift with thefield, the average energy nevertheless will correspond with atremendously high temperature of the order of tens of thousands ofdegrees centigrade. It will shortly be seen how this very high electrontemperature interferes with efiicient and desirable registration of thepulses.

A mixture of electrons and gas atoms bears a resemblance to a mixture oftwo kinds of gas. So long as the electrons remain, they will diffuseabout randomly in the gas, tending to fill up, and uniformly so on theaverage, all places where electrons are present to a less extent thanthe average amount in the mass of gas considered. On the other hand,those places which at the beginning of a diffusion process contain morethan their share of electrons will have their concentration reduced inthe direction of the average value by diffusion. Electrons diffuse veryrapidly. An electron having an energy corresponding with roomtemperature (about .04 electron volt) diffuses about one hundred timesas fast as helium. Also, the rate of diffusion of electrons rises as thesquare root of the temperature corresponding with the average kineticenergy per electron. At 30 thousand degrees, a temperature which mightbe attained by electrons in drifting with an electric field in purehelium, the rate of diffusion would be approximately 17 thousand timesthe rate of diffusion of helium. Now it happens that electrons whichcause pulses always originate in a non-uniform distribution within thegas where they are produced. A recoiling helium atom energized by acollision with a fast neutron will travel in helium and in anapproximately straight path, in a typical case, and will, at fourteenatmospheres, liberate electrons at a rate of approximately 2x10 percentimeter of travel, with an even more concentrated liberation per unitpath near the end of the travel of the energized recoil atom. Theelectrons liberated along this path are suddenly set into motion by theelectric field, if the path occurs in a mass of helium upon which anelectric field is superimposed. These electrons, however, not only driftwith the electric field, but also have random energies of motion likethe other molecules or atoms in any gas not at absolute zero. Therefore,as the cluster of electrons initially liberated by the travel of therecoiling energized atom starts toward the collector electrode, therandom motions are continuously superimposed upon the consistent drifttoward the collector electrode. The random motions have the effect ofcausing some of the electrons to arrive early and some of them tostraggle behind the main group which is drifting toward the collectorelectrode. The net result is that the corresponding electrical pulse isbroadened with respect to time; that is, it is made to last longer thanit would if there were no early arrivals and no stragglers. Now, themore intense the random motions, the greater will be the amount ofpreading, and the more numerous and the more early will the earlyarrivals be and the more numerous and more late will the late comers be.Therefore, if the electrons acquire greater energy of random motion thanthey should, the pulse of charge will not arrive in as clear cut andquick manner as would be desired. This defect will be observedparticularly in the case where absolutely pure gas of group zero isemployed in an ionization chamber or where a mixture of such gases isemployed. For such gases, the electron drifting parallel to thedirection of the field tends to impart its extra energy, resulting fromits fall through a potential difference, to the atoms of the gas bycolliding with them, and it makes one such attempt for every suchcollision. However, it is very nearly impossible for an electron totransfer energy to an atom in a ballistic encounter. This is because thelightest atoms that are known, for example, helium and hydrogen, arethousands of times more massive than an electron. An electron containingextra energy, and attempting to impart this energy ballistically to ahelium atom will lose approximately one seven-thousandth of its energyper encounter. In an ionization chamber containing helium at atmosphericpressure, it is quite easily possible to collect an electron from thespace of an inch or so thickness in a period of time so short that onlya few thousand encounters will have occurred.

Quite obviously, then, the electron would not be promptly losing theenergy that it gains from the electric field, at least not by ballisticencounters, but the energy would rise until it reached a level so highthat more eflective means of transferring its energy to helium atomswould come into play, that is, the transfer of energy will be effectedthrough inelastic collisions resulting in the transferences of potentialenergy with the helium atom from the ground state to suitable quantizedlevels above the ground state. Since, in the case of helium, as has beenpointed out before, this transition possibility only sets in at veryelevated energy levels compared with the room-temperature kinetic energyof an electron in equilibrium. The electron gas in a typical ionizationchamber will become very hot in the process of being collected by asuflicient electrical field, and will, therefore, manifest anexaggerated diffusion effect and time spreading of the pulse to anextreme degree. A molecule is no more able to absorb the energy of anelectron ballistically than is an atom, but the molecule has a muchlarger group of closely spaced possibilities of inelastic collisions,which have their onset at very low bombardment energies for incidentelectrons, bombarding energies lying in the range of .04 electron volt,more or less, and closely spaced in the vicinity thereof. Therefore, adiatomic molecule, when present, will act as a means of absorbing theenergy of hot electrons and dissipating this energy to the remainingportions of the gas. The diatomic and polyatomic molecules thus presentmay be required only in very small quantity, and are not altered ordissipated, but act in a capacity which we describe as catalytic,catalyzing, as it were, the effective transfer of energy back and forthbetween electrons and gas atoms or molecules, and promoting a promptattainment of Maxwellian statistics, which are largely deviated fromotherwise. The prompt attainment of Maxwellian statistics favors lowerelectron temperature, and accordingly, results in a minimizing of theundesirable random motions, and the production of quick pulses resultingfrom the collection of electron clusters caused by paths of recoilingatoms energized by neutrons. There is employed, in connection with thepreferred method of well logging, and it is essential to some ways ofpracticing this discovery, a gaseous mixture containing, in addition tohelium, an adequate trace of diatomic or polyatomic, but substantiallynonelectron-capturing material, to bring about the above describedenergy transfer catalysis, and promote clean-cut quick registration ofionizing events occurring in the ionization chamber.

Another embodiment of the instant invention is shown diagrammatically inFigure 5. This embodiment makes it possible to determine horizontalsensitivities of processes attributable to fast neutrons, recoilsoccurring in a particular space in the detector. The recoils which areselected are those having a path that traverses the ionization chamberin a definite manner.

The ionization chamber 147 is of the general type described inconnection with Figure 4. However it differs from that of Figure 4 inthat the portion above the screen 148 is partitioned by an element 149to divide that portion into two parts 150 and 151. The partition 149depends from the top of the housing 152. However its bottom end does notnecessarily contact the screen 148, in fact, under some conditions itmay be completely omitted.

Each of the chambers 150 and 151 is provided with its collectingelectrodes 153 and 154, respectively. These electrodes are elements inseparate circuits. Electrode 153 is connected in a circuit including aresistance 155, a battery 1'56, and ground which is connected to thecase of the ionization chamber. In a like manner, there is a circuitwhich includes resistance 157, battery 156, and ground. Potentialsdeveloped on the electrodes 153 and 154 are transmitted to amplifiers158 and 159, respectively. The output circuits of these amplifiers areconnected to a coincident circuit 160, indicated diagrammatically as arectangle. Coincident signal pulses are delivered by the output of thecoincident circuit to combination amplifier and pulse shaper 161. Fromthis element, the shaped pulses are fed to a pulse rate conversioncircuit 162, which in turn delivers a direct current, which varies inaccordance with the time-rate of occurrence of the pulses introduced toit, to the cable '163. The signal transmitted to the surface is recordedin a conventional manner in correlation with depth.

In the operation of the above described device, dense ionizing paths aredetected which cross the interior of the detector 147, in the regionbelow screen 148, and cross the plane of the partition 149, extended.Because of the fact that recoils are directed generally forward, inrelation to the movement of the neutron which produced them, thisarrangement tends to be relatively more sensitive to incident neutronswhich enter the detector perpendicularly to the plane of the element149. This transverse sensitivity also applies in the sense that neutronsincident upon the detector will be relatively neglected if their travelis chiefly along the axis of the bore hole, and will be more emphasizedif they travelled in such a manner as to enter the detector generallyperpendicular to the bore hole. Such a preference is an advantagebecause it tends to emphasize deeply-penetrating neutrons, which havegone a long way out into the strata, from the source, before returningto the detector. Such deep penetration of the neutron flux, which isemphasized, tends to produce more representative observations ofheterogeneous strata.

In order to more clearly illustrate the manner in which horizontalsensitivities can be determined, reference is made to the modified formof detector shown in cross section in Figure 6. In this form partitions164 and 165, which cross at right angles to each other, are showninstead of the single partition 149 described in connection with Figure5.

Obviously the partitions 164 and 165 divide that portion of the detectorabove the screen 148, as illustrated in Figure 6, into four equalportions 166, 167, 168, and 169. The portions are provided respectivelywith collecting electrodes 170, 171, 172, and 173.

It is to be understood that each electrode is provided with the usualcircuit consisting of a source of potential and a resistance. The outputfrom each collector electrode is fed into a separate amplifierdesignated as 174, 175, 176 and 177, respectively. The outputs of theamplifiers, in pairs, are fed into coincidence counter circuits 178 and179. The outputs of amplifiers 174 and 176 are fed into coincidencecounter circuit 178 and the outputs from amplifiers 177 and 175 are fedinto the coincidence counter circuit 179. The output signals fromcoincident counters circuits 178 and 179 are separately conductedthrough amplifier-shaper circuits and pulse rate conversion circuits toseparate transmission circuits in the cable 180 by means of which theyare conducted to separate conventional recording systems located on thesurface of the earth. Under certain circumstances, the partitions 1'64and 165 may be omitted.

In the modification of this invention shown in Figure 6, more usefuldirectional sensitivity is possible because of its ability to compareefiects in two mutually perpendicular directions. If, for example, thereis a hole with considerable directional difference, severalorientedpasses would be necessary to determine the fact, by the deviceof Figure 5, but much less effort'of this kind would be needed todetermine transverse anisotropy of the strata in the case where thedevice of Figure 6 is used. Aniso tropy can always be reliablydetermined by the device of Figure 6, if two such devices areconcurrently used, mechanically coupled with their respective quadrantsturned at 45. The exact measurement of such anisotropy is obtained, inthe case of the device of Figure 6, by subtracting the indications,delivered in the two coincidence channels containing circuits 179 and178. In the case of the concurrent use of two Figure 6 apparatuses at 45orientation, the same thing would be done to both of them and twodifferences obtained. A figure would then be calculated, which wouldrepresent a measure of the aniso- 1 where D is the difference obtainedfrom one of the Figure 6 devices, D is the difference obtained from thesecond such device, 6 is the quantity which represents anisotropy.Obviously also, Figure 6 may be modified if desired to use octantsinstead of quadrants, pairing opposites and subtracting the pulse rateconversion circuit output of pairs of these which are apart. The resultthus accomplished is to supply the two differences which may beintroduced into the above formula. Any such device or devices may beheld in the middle of the bore hole and aligned axially therewith bymeans familiar in the art.

In both the device of Figure 5, and the device of Figure 6, an advantageis gained in making the directional properties more precise, if thecoincidence circuits 160, 178, and 179 are made to be insensitive to anybut large and concurrent pulses. Also, the same purpose can beaccomplished by inserting between amplifiers 159 and 158 in Figure 5,and the coincidence circut 166 in that figure, and between amplifiers174, 175, 176, and 177 and the corresponding coincidence circuitsdiscriminator circuits for eliminating small pulses. For the purpose ofimproving the directional sensitivity of the arrangements shown inFigures 5 and 6, masks composed of sheet metal may be superimposed inelectrical contact with the screen 148 shown in Figure 5, and thescreen, not shown, which is used in the same manner in connection withFigure 6. In lieu of such a mask, it will at times be convenient merelyto close the holes in chosen areas of the screen. The open areas whichexist where either of these modifications is employed will have a formand shape related to the direction measuring purpose for which thedetector is used. For transverse sensitivity limited within a dihedralangle, the remaining open areas of the screen will be limited within theintersection of the planes of the dihedral angle and the screen. It isunderstood in the above that the line along which the planes composingthe dihedral angle limits of sensitivity intersect is coincident withthe axis of the instrument shown in Figure 5. Open sectors of 30 degreeswill often be convenient. In the detector illustrated in Figure 4 onemay use an open area lying between parallel chords of the screen disc,and distant one-fourth inch from the center of the disc. Two circularopen areas may be employed in the case of the detector of Figure 5.

These may be one-half inch in diameter, and with their centers lying ona diameter of the circular wire screen parallel to the line .joining thetips of electrodes 153 and 154. In a preferred form, the above circularopen areas will be tangent to the circular rim of the wire screen 148.

In Figure'7 there is illustrated diagrammatically an electrical systemby means of which the output from a detecting system, such as thatillustrated in Figure 5, can be treated in such a manner as to give amore accurate measurement of direction of neutrons incident on thedetector. In this figure the detecting system, up to and includingamplifiers 158 and 159, corresponds to that shown in Figure 5 andcarries the same reference characters.

The electrical system connected to the outputs of amplifiers 158 and 159serves to measure through a coin- '15 cident enabling circuit the addedeffect of coincident pulses produced upon electrodes 153 and 154 in thedetecting system. The arrangement of apparatus for accomplishing thisincludes amplifiers 181 and 182 whose inputs are connected respectivelyto the outputs of amplifiers 158 and 159. These amplifiers include pulseheight limiters so that output pulses are of uniform height.

The output of amplifier 182 is used to apply a bias potential toamplifier tube 183 by impressing a potential across the resistance 184connected in the cathode circuit of the tube. The output of amplifier181 is impressed directly upon the grid of tube 183. Tube 183 isnormally biased beyond cutoff by an amount greater than the potential ofone pulse alone, regardless of whether it is received on the cathoderesistor from amplifier 182, or whether it is received on the grid ofthe tube 183 from the amplifier 181. Thus, tube 183 cannot conduct apulse when non-coincident pulses are received from amplifiers 181 and182. However, any coincident pulses received from the amplifiers 181 and182 are, when totaled, sui'ficient to overcome the bias on tube 183 toproduce a pulse in the plate circuit of this tube. The pulse so producedis impressed on a delay circuit which comprises a condenser 185 and aresistance 186 which are connected in parallel between the plate circuitof tube 183 and ground. Since the signal from tube 187 must enable thetube 190 during the whole interval of time in which the pulse isarriving from low pass filter 196 it is necessary that the pulse fromtube be lengthened and that is the purpose of elements 185 and 186. Thedelayed signal is impressed on the grid of amplifier tube 187. Amplifiertube 187 is normally biased to a point sufiiciently close to the uppercutofi, that a relatively wide pulse is delivered to the plate thereof.The pulse produced in the plate circuit of tube 187 is conducted througha condenser 188 to a resistance 189 which is connected in the cathodecircuit of the tube 190 for the purpose of rendering this tubeconductive.

By means of an auxiliary circuit, portions of signals delivered by theamplifiers 158 and 159 are diverted through conductors 191 and 192 toseparate grids of a dual triode amplifier tube 193. The two triodes ofthe tube 193 have the same properties. The cathodes of tube 193 areconnected together and to ground through a resistance 194. The plates ofthis tube are connected together and to a source of plate potentialthrough a resistance 195. With such an arrangement of tube elements,coincident pulses on the grids of tube 193 are added together to producea single pulse signal in the plate circuit of the tube representing thesum of two pulses. This signal is conducted through a low-pass filter196 to the grid of amplifier tube 190 to produce a signal in the platecircuit when the tube is rendered conductive by the enabling circuitwhich impressed the bias potential from amplifier tube 187 on theresistance 189. The signal output from tube 190 is applied to a pulseheight analyzer one channel of which may be as illustrated in Figure 16of our copending application Serial No. 107,806. The outputs from theplurality of channels of this pulse height analyzer may be recordedorrespectively fed into pulse rate conversion circuits connected in themanner described in connection with Figure 13 of our copendingapplication'Serial No. 107,806. 'The output signals from the pulse rateconversion circuits are combined and transmitted through conductors inthe cable to a recording system located on the surface of the earthadjacent the mouth of the drill hole where they are recorded incorrelation with depth.

The device illustrated in Figure 7 enables the operator to accomplishconcurrently a measurement of the relative excess or deficinecy of'aparticular energy group of neutrons, and a better determination of thedirection of the neutrons of which there is anexcess .or deficiency withrespect to neighboring energy groups. The square hit recognitionproperty of Figure 7, limits the detect- .16 ing system to observingrecoils which are substantially collinear with the neutrons which setthem in motion, and which therefore have a direction which may be usedto measure the direction of the incident neutrons.

The need forclass recognition, such as is provided in Figure 7, arisesbecause it is impossible, in general, to provide for a wide. variety ofspecified maximum energies in a bore hole. Without the arrangement ofFigure 7, attention -of'-'directionally sensitive processes involvingsquare hit recognition would be severely limited as to the energies ofneutrons at which such processes can be carried out, limited in fact toapproximately 2.5, 12 and 14 m.e.v. It is desirable to study the generalrange of energies, because only so can one detect specific elements bytheir nuclear resonances, as has been taught elsewhere in thisapplication. The apparatus of Figure 7 thus provides for an essentiallynew and valuable result, to wit, the directional recognition of specificelements by their specific interaction with neutrons.

Another embodiment of the instant invention is illus trated in Figure 8.This embodiment lends itself to the measurment of the direction andsense of direction taken by an ionizing particle in a detector, as wellas the energy of the incident neutron.

In this figure there is shown a novel detector 199 and a source ofneutrons 198 located below the detector 199. It is to be understood thatall the apparatus of Figure 8 is to be'assembled within a housing thatis adapted to traverse the bore hole.

Detector 199 consists of a housing 200 which encloses an iouizablemedium such as a mixture of hydrogen and helium, but predominantly oneor the other, under a pressure of from 10 to 500 atmospheres dependentupon the dimensions of the detector, and the voltage applied thereto.Inside the detector, and disposed in the ionizable mixture, are aplurality of ring shaped electrodes. These are divided into two groups201 and 202 by a screen 203. The ring electrodes are vertically spacedfrom each other by substantially uniform intervals of distance. Alsodisposed in the detector in such a manner that they extend inwardly ashort distance from each end, are electrodes 204 and 205. The regionabout electrode 204 is divided from the region of the ring electrodes202 by a screen 206. In like manner the screen 207 divides that portionof the ionization chamber immediately surrounding the electrode 205 fromthe region of the rings 201. The housing 200 of the detector is groundat 208. Conductors 209 and 210 make connections respectively between theelectrodes 204 and 205, and external circuit elements to be described.

Conductor 209 forms, with resistance 211, battery 212, and ground 213,an electrode circuit for the top electrode 204. In the same mannerconductor 210 forms, with resistance 214,battery 212, and ground 213, anelectrode circuit for the bottom electrode 205.

Signals produced across the resistance 211 are amplified by the pulseamplifier 215, and fed to a threshold circuit 216. Threshold circuit 216serves to block small signals such as those that are produced by thedissipation of gamma rays.

Any signals that extend over the threshold for which circuit 216 isadjusted are allowed to pass and be conducted to a pulse sharpeningcircuit 217, which may be a difierentiator. The signal output from 217is introduced into a pulse shaping circuit 218 which consists of meansfor lengthening the pulses with respect to time but not changing theirheight. The pulses so shaped are then impressed upon the grid 219 of adouble triode amplifier tube 220, for a purpose to be described later.

Signals produced across the resistance 214 are amplified bythe pulse.amplifier 221, and fed into a threshold circuit 222. Threshold. circuit222 functions in the same manner asthatdescribedfin connection withthreshold circuit 216. Signals from the threshold circuit 222 are fed111m an inverting circuit 223. After inversion the signals. areconducted to arpulse sharpening. circuit. 224;. Which-may be a.differentiating. circuit. The sharpened pulses are then fed into a pulseshaping circuit. 225which lengthens the pulses with respect to. timebut. does not increase or decrease: their height. Theshaped. pulses arethen impressed upon the. gridf226 of the. amplifier. tube 220. It. istov be understood. that both halves of' the double triode will. havesimilar properties. Obviously-,. tube 220 can be replacedb'y two:triodeshaving matchedcharacteristics; The cathodesofthe double triode2'20 are connected together, and through. resistance 227" to ground. Theplates of this tube are connected together to forma single outputcircuit. Plate potentials are supplied. to both plates through theresistance 228} Both halves of 'tube 221T are biased! to apoint'which'is at substantially the center oftheir linear amplifying range. Anysignal flowing. in the plate circuit from tube 220 W-ill'berepresentative" of the difference between signals impressed upon grids 219'and. 226". The polarity of this signal wilhde'termine which. ofthegrids'hadthe' greater potential impressed uponitby their-circuitsdescribed'irmnediately ab'ove. Thedi'fi'erence betweenthe'potentials-applied to the grids 2 19 and 226willfdet'ermine" thedirection andsense' of direction" of"ioniz'ing"p'articles passingthrough; both halves of the ionization chamber within the range oftheelectrodes.

The density of ionization occurring" along the path" of a heavy-particlerecoil'in an ioni zabl'emedium" is nonuniformand is much'greaternearth'e endofth'e'path. Hence, if an arrangement ofapparatus' isprovided'for measuring theioniza'tion of the -earl'yand"late-parrot the"path separately, obviously, the ionization appreciated from'thelate partof the path'will be greater" than" that from the earlypart of the path?If tlieionizablemedium is" a gas which does not attach"electrons andwhich has a means provided for maintaining thefree electronsata'relatively. low temperature, it is" possible to collect thefreeelectrons occurring along the heavy 'particle-path" sufficientlyquicklywhat their time'arrival'at" the collector" electrode Willbe'a'fithfulrepresentationof theirtime of' liberation: An exception tothisisaparticlepathwhich" lies alonganequipotential-planednthedete'ctofi These paths, however, are" ignoredin" the appa-ratus here d'e-- scribed; The collection"of"electrons*fro-m" the 'late par-t of thepath will not only containmore total energyhut will also i be characterized" by 'a-'- higher rateofarrival or energy-at the electrode thanffom "the 'e'arl'y part-of thepath;

The detector 199 shown in- Figure'8 makes' pos'sible the results statedabove Particle-pathsarising-in" the space-- enclosed by ringelectrodes-201 and ending 'in the spaceenclosed by ring electrodes 202willthen 'cause a positive pulsein the plate circuit of'tube 220 =a11dif 'the he ginning and end were reversed withrespect to these-two spacesthe pulse appearingintheplate-circuit of tube 220 will then be negative;

The output signal from tube' 220, 'regardless ofits polarity, isimpressed upon the grid of an amplifier tube 2291 The function of thissignal will be described in conn'ec-- tion with theauxiliarycircuits;a-deseription of which will follow;

A portionof the signals passing*throu'gh thethreshold circuit 216 I arediverted to an auxiliary circuit which'in cludes an amplifier 23'0 and-aclipping and shaping circuit 231; The output pulses from circuit -231'wil1 thenbeconducted to and=impressed upon, the 'gridof tube 232. Inoperation this tube functionsjointly with' theauxiliarycircuit whichconducts aportion of the signals-diverted from the output of thethreshold circuit'222 'to an ampli fier 233. The output-of'amplifier 233is-fed into a clip ping and shaping circuit 234 from -'wh'ichit isconducted" to" the cathode circuitof tubes 229 and 232 'and thereimpressed across the resistor 2315 Tube 232 is normally conductive. Tube229 is nor-- lg normal potential on'resistance 235 due to the conditionoflthecircuit through-tube 232 and to the bias placed on'the grid oftube 229; However, when the potential supplied by the clippingandshapingcircuit- 234 is applied across the resistor 235, anda pulseimpressedfrom the clipping and shaping circuit 231 is appliedsimultaneously onthe gridof tube 232, then the signal from tube 220Which-is impressed on the gridof tube 229'causes a pulse ofcurrenttoflow in the plate circuit of tube 229; This is true because tube 229has. been rendered conductive by the simultaneous arrival of pulses fromcircuits 231 and 234. These output pulses will have individual polaritydetermined by the polarity of the pulses from tube 220'. The enablingpulses impressed across the resistance235 and on the grid of tube232*are of lower frequency than the signals received on the grid of tube229? This situation is made to exist'in order that. separation of thedesired'signals can be made from'the enabling" pulses by means of "asuitable high-pass filter236. The pulsesfio'wingfrom the high-passfilter236' are amplifiedand clipped by the amplifier 237. The'pulses flowingfrom-amplifier 2'3'7may be. either positive or negative in polaritydependingupon the outputpulses from the tube 220. Pulses of positivepolarity will flow through the condenser 238' m the grid of tube 239,while negative pulses will flow through: the condenser 246/to the-gridof tube 241. This is made possible by maintaining tube 239 in a normallycutolf" condition and tube 241 in'a normally conducting condition suchthat tube 239'wi11 pass only signals which are'positive in polarity andtube 241 will pass only signals which are negative inpolarity.

When 'a-positive signal is impressed upon the grid of tube'239"the tubetends to become conductive and negative pulses are produced on the plateoftube 2391 These ,negati've'pulses are conducted through condenser 242to the grid of tube 243. This tube is normally conductive. This negativepulse causes a drop in theplate current of tube 2'43, resulting in'atendency to produce a drop in potential across resistor 244. Resistor244 is a common" .cathode resistor for both tubes 239 and.243. This reduc'ed'p'otentialacross resistor 244 causes the plate current in tube239 to increase still further. This, of course, causes a furthernegative signal on the plate of'tube' 239. This positive feed-backarrangement continues to operate untiltub'e 239 reaches saturation andtube 243 becomes noncondu'ctive. When a portion of the charge inthecondenser"242 leaks ofi through the bias resistors, the circuit is"rapidly restored to normal by the positive feedback just'described. Thepulse produced'by the circuit'at'the' plate'of' tube 239"is" arelatively low frequency pulse whicliis conducted through condenser 245to'the cathode of tube 246i Tube246 is normally biased beyond cutofi,but this negative pulse impressed upon the cathode," causes thetubetobecome conductive. The tube now can receive signals on the grid andfaithfully amplify them. A

manner in which signals impressed upon'the grid' of this tube areproduced and the function that they serve will be described later. 7

Aspointed out above, pulses fromthe amplifier 237" are delivered to thetube 241. Tubes 241 and 2'48 operated inWhemanner described inconnection'with tubes 239 and tubes 243; The only difference is thattubes 241 and248 are'triggered by negative-pulses whereas tubes23'9"and243 are triggered by positive pulses. The pulses produced in theplate circuit of tube 248 are conducted through a" condenser 249 tothecathode of tube 247. Tube 247 in'the same manner as described inconnection with tube 246, is normally biased beyond cutoff and thesignalimpressed upon thecathode resistor serves'to condit'ion' the tubefor'the reception of a'signal on its grid. Tube246 or 247 will functionto produce a pulse in their plate circuits, dependent upon the? polarityof the signalproduced by tube 220. The signal that is desired to berepresented in the plate circuit of either tube 246- mally biased toa"point"'beyon'd"cutofi;bynieausoftlie" 75, 01-247 is. a pulse whichwill represent the total energy.

of the ionizing process which occurs in the detector. To accomplishthis, a portion of the signal from the threshold circuit 216 and aportion of the signals from the threshold circuit at 222 arerespectively impressed upon the grids 2'50 and 251 of the dual triode252. It is to be understood that both halves of this tube will havesimilar properties, or this tube may be replaced by two independenttriodes having similar properties. The cathodes of this tube areconnected together and to ground through resistance 253. The plates areconnected together and to a source of potential through a resistance254. A coincident pulse delivered by the threshold circuits 216 and 222when impressed upon the grids of tube 252 will add together to produce asignal in the plate circuit of this tube which is proportional to thesum of the amplitudes of the coincident pulses. The signal is conductedthrough an artificial line 255 which has as its purpose to delay thesignal a specified length of time. The delayed signal is then conductedto the grids of tubes 246 and 247 through condensers 256 and 257. Thissignal represents the energy of the incident particle. This signal isimpressed upon the grids of both tubes 246 and 247 and will be conductedthrough only one tube, dependent upon which tube has received anenabling pulse on its cathode. The tube which receives the enablingpulse depends upon the polarity of the signal from tube 220 and henceupon the direction and sense of direction of the path of the ionizingparticle in the detector.

Pulses flowing in the plate circuit of tubes 246 and 247 are conductedthrough high-pass filters 258 and 259, respectively. The outputs fromthese filters are conducted through separate circuits to recorders whereseparate records may be made in correlation with depth.

The apparatus shown in Figure 8 represents a novel and valuable Way ofpracticing the instant invention. The new element of information whichhas been added is the sense of direction of neutrons. Heretofore, thedirectional means which have been disclosed offer no opportunity todetermine this fact, offering only the possibility of knowing that aparticle took one of two opposite courses.

This desirable result enables the operator to select neutron particlesentering with a component in the direction opposed to the sense which aparticle would have if it travelled on the direct line of travel fromthe source. By selecting incident neutrons which are so directed andwhich have high energy, one is able to measure that portion of the fluxof neutrons derived from the source which has penetrated very deeplythrough the strata, and is therefore adapted to give a valuableimprovement of accuracy in the measurement of very heterogeneous strata.

A modified electrical system for determining direction, sense ofdirection, and energy of the particle producing a process Within thedetector, is illustrated in Figure 9. Conductors 26% lead from adetector, not shown, such as that illustrated in Figure 4. The top onemakes connection with electrode 27 and the bottom one makes connectionwith the case or ground.

It is to be understood that the detector will be provided With theconventional electrode circuit which includes a resistance and highpotential source. Pulses from the detector are conducted to theamplifier 261 where they are amplified and delivered to a delay line262. The pulses passing through the delay line 262 are transformed bythe radio frequency transformer 263 and impressed upon the grids oftubes 264 and 265 in the manner shown. The plate of tube 264 isconnected to a source of potential indicated as B+. The plate of tube265 is connected to the same source of potential through a resistor 266.The cathode of tube 264 is connected to ground through a resistor 267.The cathode of 265 is connected to ground through the resistor 268. Thecathode of 264 is connected to the plate of tube 265 through condenser269 and a lead is brought out to 20 the condenser 270. Condenser 270 maybe connected to a pulse height discriminator circuit or to a pluralityof such circuits.

7 A portion of the signals conducted from the detector are divertedthrough an auxiliary circuit. Conductors 271 conduct the divertedportion to amplifier 272 where they are amplified and are delivered tothe derivating circuit 273. The derivated signals are then introducedinto threshold circuit 274. The threshold circuit 274 takes the formshown in Figure 10. In Figure 10 the input of the derivating circuitcomprises the conductors 275, by means of'which the signals areconducted through a condenser 276 and resistances 277 and 278 to thegrid of a tube 279. The plate of tube 279 is connected to a source ofpotential indicated by B+. The cathode of this tube is connected toground through a resistance 280. A resistance 281 having one endconnected to ground, has its other end connected at a point betweenresistance 277 and 278. Tube 279 is followed by a tube 281. The grid of281 is grounded for radio frequency through condenser 282. The cathodeof tube 281 is connected to the input end of the resistor 277. The plateof tube 281 is connected to a source of potential through a resistor284. This plate is also connected to the cathode of tube 279 through thecondenser 285. The output of this circuit is transmitted through thecondenser 286 and ground terminal 287 is the other output terminal. Thethreshold circuit shown in Figure 10 passes positive pulses greater thana certain magnitude through tube 279 and negative pulses greater than acertain magnitude through tube 281.

Again referring to Figure 9 the output signal from the threshold circuitillustrated in Figure 10 is fed through low-pass filter 288 to theamplifier 289. The amplified signal is then introduced into the shapingciredit 290. After the signal has been shaped in circuit 290 it is fedthrough a double pole, double throw switch 291, to the secondary circuitof radio frequency transformer 263, passes through the battery 292. andis then divided. A portion of the signal flows through the resistance293 and the remaining portion flows through the resistor 294. The signalfed through the resistances 293 and 294 does not appear in the output ofthis system when there is no signal coming through the radio frequencytransformer from the delay line 262. Likewise the signal transmittedthrough. the radio frequency transformer 263 does not appear in theoutput terminal leading from condenser 270 when there is no signaltransmitted by resistances 293 and 294. When a suflicient signal istransmitted through the resistances 293 and 294, uniformly shaped inelement 290, and when concurrently there arrives a signal commencingafter the beginning of the shaped signal and ending before theconclustion of the shaped signal which is the other signal comingthrough the radio frequency transformer 263, then a signal will appearat the terminal of condenser 270 and it will faithfully represent thesignal transmitted through the radio frequency transformer 263.

In operation, signals transmitted through the upper portion of thecircuit, which includes the amplifier, delay line, and radio frequencytransformer 263, will be impressed upon the grids of tubes 264 and 265.These tubes will conduct pulses to the output circuit only when theyhave been enabled by a signal which has been conditioned in the lowercircuit, and delivered to the grids of these tubes, as previouslydescribed, through the resistors 293 and 294. The enabling circuit isprovided for the purpose of causing transmission to the output, ofaparticular class of information and excluding unwanted classes ofinformation.

The type of classification which is accomplished by the enabling orlower circuit of Figure 9, has as its purpose, as has been previouslystated, the recognition of direction and sense of direction of particleswhich cause the ionizing paths in the detector to which the circuitscuit of Figure 9 is madeipo'ssible by& the fact: that theelectrical'signals which occuri'intthe radiation detector connected tovthe input oflthis systemzare: different for ditferent direction,and'sense of direction-iof tlie: particles whichcause the ionizing.paths'finathe" aforesaid detector.

'Fhe-casewhichiis-most useful forithe employmentof .the

classificationprocesses accomplished by" the lower cire cuit of Figure9, is that .inzwhich the: ionizing process occurring in the circuitofielectrode zfl, was caused by an ionizing path generated by aparti't'ale in: the radiation detector, which moved in a.v direction;havingta sub'stantial component parallel to the aXis'ofiFth'e detector;

For the above class of processes, corresponding with incident neutronsdirected generally along the axis of the detector either upwardly ordownwardly, there are two kinds of electrical signals which appear onthe electrode 275 For neutrons'movingf along the axis, and movingdownward, the resultingrecoil 'atomsgenerally-produce at the electrode27 signals like that" represented in Figure 11A. For neutronsincidentupwardly upon the detector of radiation, the resulting recoilsgenerally produce electrioal'signals on the electrode 27 like thatillustratedv in Figure-1i1-F. The signal illustrated in. Figure llAtakesthetform-shown in:Figure 11B after passing through the.

slightly greater than the sumrof-thervoltage of batteryv 292: and the:normaloperating bias-of. tubes 26'4rand 265. The signalscorrespondingwith a=recoilcaused by a neutron incidentupwardly along? theaxis of thedetector off'astineutrons' are similarly transformed andresult in asimilar: fiat'-toppedsignalr 1 1], but of'opposite polarity. Thepolarity which occurs in diagrams llEand' 111 is, in each case; broughtabout by thefact thattheintegral of the highspeaksof the: derivatoroutput has opposite polarity; Thetransient which occurs between thesepeaks, and

after them, which. would otherwise cancel the value ofintegral, iseliminatedthrough the action'of the threshold circuit-271. Circuit 271'acts to: ignore the portion of the derivator output between the dotted.lines in diagrams ofiFigures'llB and 11G; The action of. the lowercircuit oftFigure. 9 will enable the passage of pulsesthroughtheuppericircuit, only in: those instances. where the pulsesgenerally'have'the form. of-Figure llA-if the inversion switch291;.isin'theleft' hand position. If the inversion switch 291. is in.the; right. hand position, the enabling circuitiwill thent. transmit:signals only when the pulses received. from'thec'. electrode 27 havethe:form shown in Figure: 11F. The-waves shown indiagrams E; and J of Figure11 differ. inthat they. will have different'times-of onsetlb'y aniamountwhich isofthe order of the duration oflthe pulsesshown. in Figures 11A:and-11F. T-o-overcome the difiiculty. caused by this inequalityof timesof onset. of wavesaillustratedin: Figures 11B and 111,. At shown:iIIWhBLfigUIGL is made two or threatimes longer than therdurationof timeinxwhichthepulses shown in Figures 11A and lflF exist. Thedelayintroduced by the delay 1ine 262d's made sufficientto cause: apulse travelinggalongtthe: upper'circuitof Figure 9-to the radiofrequency transformer 263, to arrive just after the time of commencementof the enabling pulse shown in-Figure 1&1 E; twliich" is:theinlasttarriving-uofz thentwo possibilities.

'Ehe pu1ses transmittediintheiupper circuit will be faithfullyrepresented and transmitted to ther outputv terminal of condenser-270,because of thechosensmooth top. of-the enabling pulse, and. becauseofthefact that it is made to offer an equal voltagewhich does not varythrough the 1 time that 'theuppercircuit is enabled. Theenabling pulsetherefore renders-the tubes 264 and 265 continuously andequallyconductiveduring the enabling interval, which includes the timeofarriva-l-of the desiredevent from the radio frequency transformer 263,regardless of whether it is anevent corresponding withaFigure 11A,.downwardly incident neutron, or an event: corresponding to- Figure 11F,upwardly incident neutron, selected by suitable setting of:switch 291.

In Figure 12 there is. illustratedan electrical system that is adaptedto be employed-with certain apparatus or combinations of apparatus shownin therprecedingfigures of drawings for the purpose of discriminatingbetween pulses on the basis oftime duration and. faithfully amplifyingand recording the pulseshaving a specified time duration incorrelation-withdepth.

Pulses from a. detector, such asthat shown in- Figure 4 are amplifiedby=the amplifier 295 and-introduced-into a threshold circuit 295a andclipper circuit 296. The clipped pulses are then differentiated by thedilferentiator circuit 297. The action of the differentiator is to'produce. a. signal such as that illustrated a298,. that. is, one havingnegative. and positive pips- The circuit built around tubes- 299 and300will'besrecognized as atrigger circuit,.whichis-.triggered by theleading pip-of thesignal shown at 298 I to produce'a-pulse ofspecifiedheightand duration. The

duration of thispulse is determined'by the lower limit. of the range ofpulse duration selection desired. The pulse produced by the pulse.generating circuit is amplified by: an amplifier 301andthen-differentiated. by ditferentiator 1 302to produceasignal such asis-illustrated at 303.. It is.

to be notedthat thissignal iscomposed ofa negative pip followed-by apositivepip. The positive or trailing pip triggers a blockingoscillatorcircuit. which includes the. tubes 304 and 305,..connected. inthe'manner shown,.andf. the transformer 306. The blockingoscillator whentriggered, produces anarrow pulsewhich is used as anenabling. pulse in.the. cathode circuit of tube 307. The. width of this pulse determinesthe range of. pulse, duration selection. Tube 307 is normally biased tocutoff andthe enabling pulse conditions it to become conductive when.asignal of proper polarity is impressed'on itsgrid. Ifa: signalcorresponding to that illustrated" at 298' occurs while the tube 307 isenabled, the positive pip thereof, when impressed on the grid of' tube307, will cause a pulse to be produced in the plate circuit thereof.This pulse will be impressedon'the grid-of aatube 308 which will, withtube 309 and the associatedelements, form a triggered pulse generatingcircuit. When triggeredby a pulse from tube 307' this circuit willproduce a pulse having specified height and duration. This pulse isimpressed across the resistance connected in the cathode circuit oftubes 310 and-311 and'servesto enable or condition these tubes forconduction when signals. are impressed on their grids.

A portion of the output signals from amplifier 295 is diverted through adelay circuit 312'and amplifier 313 to the transformer 314. The signalfrom transformer 314 is conducted through condensers 315 and 316' to thegrids of tubes 310 and 311. Tubes 310 andv3ll are normally biasedbeyond'cutoif. The pulse from. tube 308'is negative in-sense and justsufiicient in height. to cause a fall in potential of their cathodessuch. that the tubes310 and 311 will be in the-middle of theirconductingrange. The duration of this pulse is sufficienttomaintain-thetubes in. a conducting state until the pulse. arrivingfrom trans-- former 314- isfinished. The output of tubes 310 and 311 istaken from the push-pull plate-to plate highfrequencytransformer 317.This output signal-is a faithfully am. plified representation-of thepulse arriving from trans former 314, and because of the push-pulLconnection;

contains none of the enabling pulse impressed on the cathodes of thesetubes. The output of transformer 317 is impressed upon the tube 318which is operated as a cathode follower and hence the output of thistube is taken between the cathode and ground. The output of this circuitmay be fed into a pulse rate conversion circuit and conditioned forrecording in correlation with depth of the drill hole in the mannertaught in an earlier part of this specification.

The uses of the device of Figure 12 are very numerous in neutron welllogging. Among other things, the device of Figure 12 enables theoperator to concurrently determine the energy and direction of neutronsincident upon the detector. If the devices previously described aresuitably combined with the device of Figure 12, the operator maydetermine concurrently the energy, direction, and sense of direction ofneutrons incident upon the fast neutron detector. Direction, as usedabove, means simply the angle of incidence of the neutron upon thedetector with respect to the axis of the detector. Geographic sense ofdirection may be added, when desired, by superimposing on the pulseselection described in connection with Figure 12, selection like thatdescribed in connection with Figures and 6 which gives geographicdirection selectivity.

By use of the device of Figure 12 it is also possible to accomplish aclassification of the types of ionizing tracks occurring in the chamberof Figure 4, according to the types of ionizing particles which causethese tracks. How all these purposes are accomplished will becomeclearer from a consideraiton of Figure 13.

In Figure 13 there is illustrated at a a path of a heavy ionizingparticle, such as a recoiling helium atom. Because the space inside thedetector rings 76 is occupied by a uniform and straight electric field,so polarized as to draw the electrons upward, the row of electronsreleased by the recoil may be thought of as being uniformly translatedupwardly at high speed, but relatively undistorted after being formed.For purpose of illustration, there is shown at b the location of the rowof electrons at time t after it was formed at a, and at c, the positionof the row at a still later time t Because of the concentrated nature ofthe electric field around electrode 27, electrons are appreciated at aconstant short interval of time after they pass through the screen 77independently of where they enter it. The electrical pulse generated onelectrode 27 will, therefore, substantially faithfully represent therate at which the electrons passed through the screen, during aninterval of time in which a path defined by electrons arrives in frontof the screen in a position to pass through it. Because of the uniformnature of the translation of the electron paths and the uniform mannerin which they pass through the screen and are eifective on electrode 27,a given duration of electrical pulse At will always characterizeionizing paths occurring in the detector. Such a path could be thoughtof as beginning and ending on any two parallel planes 319 and 320 whichare perpendicular to the electrical field in the detector, providedthese planes are separated by a distance d When using a detecting systemsuch as that illustrated in Figure 13 it is desirable to defininitelylimit the class of directions of recoil paths caused by recoils of aspecific energy group whch are detected. Such a definite limitation isachieved by superimposing on the signals received from a detector, suchas is illustrated in Figures 4 and 13, two selecting processes. Theseselecting processes are accomplished by connecting to the output of thedetector a circuit of the type shown in Figure 12. In such a case therecoils, which result in electrical pulses benig appreciated in therecording circuit, will correspond with paths in the detector which hada fixed angle with direction of the axis of the detector. The aboveselection processes fix this angle because they amount to a measurementof the total length of the particle path plus a measurement of itscomponent parallel to the axis. Since the amount of ionization may bemade determinative of the path length and the pulse height isproportional to the total amount of ionization then the pulse heightmust necessarily be a monotone increasing function of path length. For agiven particle, and in a given atmosphere there will be a uniquelydetermined particle path length corresponding with each energy. Likewisethe component of the recoil particle path parallel to the axis is known,being determined by the selection of At and interplanar distance d inthe circuit of the type shown in Figure 12.

The selection angle determined, as explained, is calculated from theequation where 0 is the angle between the direction of a recoil path andthe direction of the axis of the detector;

d is the interplanar distance shown in Figure 13; and

h is the ionizing particles path length, determinable from the pulseheight.

The point proportional counter, familiar in the art, and the straightfield ionization chamber, also well known, have been combined to producethe apparatus shown in Figure 4. The combination is accomplished by theprovision of the screen 77, which takes the place of a collectorelectrode in the straight field chamber, and takes the place of part ofthe wall of the proportional counter. By placing the open part of thescreen at a region of maximum and constant sensitivity, and designing itas an electrostatic shield, the combined detector becomes a proportionalelectron impulse ionization chamber as well as a proportional counter.Because of the larger volume of the straight field space below screen 77for collection, much greater sensitivity is secured than is available inpoint counters. Also, as has been pointed out, the arrangement of Figure4 enables the faithful electrical representation of successive portionsof an electron track caused by a particle path, and in the order inwhich the successive portions collected were initially nearest to thescreen 77. Other uses and advantages of the device shown in Figure 4will, with the above teaching, occur to those skilled in the art.

A novel electrical impulse detector which employs ring electrodes toshape a portion of the electric field within it is also described. Thedescription also sets forth a system of analyzing the recoils caused byneutrons to determine the energies of the neutrons that caused them asan indication of the presence of specific elements. This last describedsystem is also disclosed as applied to a well logging operation.Alternative systems for those enumerated above are also described. Agroup of detectors responsive to specific elements at defined directionsfrom the wall of the bore hole are disclosed, some being responsive tohydrogen only, and others being responsive to any desired chemicalelement. Selected detectors of this group are capable of being used fordeep and accurate investigation of very thin strata. The presence ofspecific elements in the formations can be determined by a systemdescribed above which enables the ascertainment of the kind of particlewhich produced ionization in a detector of fast neutrons, the length ofionizing path and the energy expended in producing the path.

It is to be understood that this invention finds specific application towell logging in that it affords means and apparatus whereby petroleumand other valuable substances can positively be located in situ. It hasgeneral application to the measurement of the influence of the nuclearparameters of various chemical elements upon a flux of fast neutrons.

We claim:

1. A method of detecting radiation that comprises subjecting a firstregion of a confined ionizable medium to said radiation, translating,the electrons thereby produced with substantially no further ionization"through said;'" first region into asecond region of saidlmedium,accelerat ing said 'electrons'insaidsecond region'to produce furtherionization and thereby multiply the number of'electrons, collectingsaidmultiplied electrons, and measuring the resultant electrical pulses asindicative ofthe incident radiation;

2. A method of detecting radiation thatcom'pri'ses'suh jecting a firstregion of'a confined ionizable gas to said radiation, translating as agroup the electrons thereby produced with substantially no 'furtherionization through said first region'into a second region of saidgas,accelerating said electronsin said secondregion"tomultiplyelec trons bygas amplification, .collectingsa'id multiplied electrons, and measuringacharacteristic of the resultant electrical pulses as indicativeoffaquality 'of the incident radiation.

3L A method of measuring a flux of'fast neutrons'which' comprisesproducing witlfthe said neutrons recoil particles of 'the' atomic nucleiof a detection" medium, deriving fromsuccessive'parts ofthe'paths of thesaidrecoil par ticles in said medium differentelectrical signalsiandelectrically'combining said signals to indicate the direction of travelof the said recoils 'andhence' of the neutrons which caused them.

4'." A methodof detecting fast neutronsthat comprises producing withsaidneutrons recoil'particles of'the atomic nuclei 'ofa confinedionizable medium'divide'd into at least two regions, said recoilparticles thereupon producing a' patlrof electrons in'said ionizablemedium; separately collecting'the'electrons'produced in each of saidregions, thereby forming electrical" pulses; and measuring; the pulsescoincidcntly collected inatleast two of said regions as ameasureof theoccurrence of recoil particles'having paths partly in at least"tworegions and'hencea measure of "the occurrence of neutrons incident 'uponthe detectorindirections corresponding generally. with the directionsof'the said'recoil paths.

5. A methodof'detecting fastneutronsthat'comprises subjecting anioniZablmedium to the 'neutrons,"-producing'electrons inpaths definedby recoilparticles produced: by said neutrons in said medium; translating withsubstantially no further "ioni-zationthe electrons "defining 'said pathsthrough a'uniform' electric field'intoa region ofnon=uniform'electricfield, saidnon-uniforrn electric newaccelerating,the'electrons inp'aths leading to a"point of collection,and collecting the electrons thus producing electric pulses.

6. A method of detecting radiation that comprises subjectingarrzionizable medium 'to sai'di radiation ,';separately collectingthe'electrons-thereby-produced ineach of two regions of said ionizablemedium, deriving electrical pulses fromelectron collection inbothwregions coincidentlyl as indicative of the occurrence of electronpaths-partly-irr both regions and hence of incident radiationgenerally-- in the directions of the electron-paths; separating;-thederived pulses in accordance with which regionv had the greater numberof electrons coincidently produced therein as indicative of! th'esenseof? direction, and measuringttheseparated pulses as indicative ofincident radiation having particula-r directionsand senses of direction.

7,. A method of detectingradiation that comprises subjecting at-confinedionizablemediumto radiation to pro duce therein: ionization evidenced byelc-tronswhich define charged parti'cle paths, the sense ofdirectionofsaid paths being distinguishable by the lesser density ofelectrons at the beginning of each path; uniformly translating theelectrons defining said paths toward an electron collector; collectingsaid electrons in the order of their initial distance from saidcollector, the nearest electrons being collected first, therebyproducing electrical pulses having shapes indicative of the sense ofdirection of said paths; separating said pulses in accordance with pulseshape; and measuring said separated pulses.

' A method of detecting radiation thatcornprises sat jecting aconfinedionizable"medium to radiation to pro duce therein'ion'izationevidenced by electronswhich tie-'- fine charged particlepaths, uniformlytranslating the'electrons'defining" said paths toward an electroncollector,

9. An apparatus for producing a neutronlog-of a well" that comprises asource of fast neutrons; means for traversing the well with the'sourceto eifect'bombard mentof the formations with fast neutrons; means'disposed adjacent said"sourceand adapted for movement therewith' fordetecting fast neutrons which have been diffused "by the formations. andreturned to the well, said" detecting means comprising a housing; an"ionizab'le mediumlin saidh'ousingga collecting'electrode, a'plural ityof ring electrodes; a' screen dividing the region of the ringelectrodesfrom the region'of the collector electrode; means for placinga'potential'between the collector electrode and" the housing and'screen;and meansforplacing, potentials between individual electrodes and thescreen to provide a substantiallyfiuniforrn'field in" said" region of"ring electrodes; means for recording" signals resulting from saiddetection in correlation'with the depth at which detection occurred; andmeans for transmitting the signals from the detectorto'the recordingmeans.

10. An apparatus'for producing a neutron'log of a well"that;comprises asource of fast neutrons; means for traversing the well'withf the sourceto eifect bombardmentof the formations with fastneutrons;tmeansdisposed" adjacent said source and adapted for movement therewitlifor detectingzfastineutrons which have been" diffused by theformationsand returned to the well, said'detecti'ng means being as recited inclaim 9 further characterized by a plurality of collecting electrodes,and means for connecting the collecting electrodes'in pairs to anexternal circuit; meansfor recording signals. resulting from saiddetection in correlation with the depth at which detection occurred; andmeans for transmitting' the' signals from' the detector to the rwordingjmeans.

11. An apparatusfor'producinga neutron log of'a we'll that'cornpris'es asource'of fast neutrons; means for traversingthe"wellwlththesource toeffect bombardment of the formations witlrfast neutrons; means disposedadjacent said source and adapted for movement therewith for detectingfast neutrons which have been diffusedby' the formations and returned tothe well, said detecting means having a pair of collecting electrodesadapted to separately collect electrons produced by the first and lastportions of'ionizing particle paths; means fordistinguishing'the firstportion from the last portionymeans forproducinga' signal indicativeofthe sense of? direction of'the ionizing particle; means for recordingsignals resulting fronr said detection in correlation with the depth atwhich detection occurred; and means for transmitting the signals fromthe detector to the recording means;

12; An apparatus for producing a neutron log of a wellth'at'comprises asource offast neutrons; means for traversing'the' well" with a source toeffect bombarding with the formations of fast neutrons; means disposedadjacent saidsource' and adapted for movement therewith fordetecting'fast neutrons which have been diifused'by the formations andreturned to the well, said detecting means comprising a housing, anionizable medium in said housing, a point collecting electrode, aplurality of ring electrodes, a screen dividing the region of the ringelectrodes from the region of the collector electrode, means for placinga potential between the collector electrode and the housing and screen,means for placing potentials between individual electrodes and thescreen to provide a substantially uniform field in said region of

